The activity of endotoxic lipopolysacchardes seems to be limited to components and structural features of the lipid moiety. Hypothetical structures for the active sites were proposed. Verification of these working hypotheses is one of the aims of continued research. Furthermore, it has been found that highly purified endotoxin preprations can enhance host resistance against TA3 tumors. Studies of the events leading to the development of such nonspecific tumor resistance are outlined.